The present invention relates to hydraulically actuated devices, and more particularly to a locking mechanism therefor.
While not intending to be limited thereby, the present locking mechanism will be described in connection with a ram type blowout preventer inasmuch as the locking mechanism is particularly useful therewith, but the present mechanism is useful with various types of hydraulically actuated devices.
Ram type blowout preventers include a pair of rams which are actuated to provide reliable sealing around drill pipe, across an open hole and for shearing drill pipe in various drilling situations. The rams usually are hydraulically actuated, and a number of these devices are provided with a mechanical assembly to lock the rams in the closed position once the rams have been actuated to the closed position. Usually no additional hydraulic force is required to maintain the rams in the locked position, and they generally can be opened only by the application of an opening hydraulic pressure.
Various types of mechanical locking devices have been provided. U.S. Pat. No. 3,242,826 to Smith illustrates a mechanical locking device for a ram type blowout preventer. The locking device uses a plurality of locking segments associated with the piston of the hydraulic assembly. A locking cone moves the locking segments outwardly, and the segments expand into a locking shoulder in the cylinder wall of the hydraulic cylinder/piston assembly. Locking devices of this nature have several drawbacks, among them being the fact that the locking segments score the cylinder wall, the segments cause increased friction, and the diameter of the ram actuator piston must be increased to provide sufficient force to move the locking cone and release the locking segments.
Other patents disclose various types of locking devices for ram shafts, but such devices are relatively complex and have various disadvantages. Among these is the device shown in U.S. Pat. No. 4,024,800 which uses a resilient catch member to lock a piston when the tubular locking cap is in a locking position. U.S. Pat. No. 3,186,163 illustrates a barrel detent in which a resilient element is compressed and a cam sleeve moves in a manner to move fingers out of engagement with a collar. U.S. Pat. No. 3,451,313 illustrates a locking actuator device having fixed fingers extending in a manner to engage a piston, and the piston includes a resiliently mounted locking plate which prevents the piston and the fingers from becoming disengaged until appropriate pressure releases the locking plate. U.S. Pat. No. 3,003,471 discloses a stroke and locking device using an auxiliary reciprocal sleeve piston which maintains fingers in locking engagement. Other patents of general interest include Allen U.S. Pat. No. 3,208,357, King No. 3,386,338, Fredd No. 3,395,618, and Walker No. 3,580,140.
While the prior art discloses various approaches to the problem of locking a hydraulic ram shaft, none appears to provide a positive lock by using positive locking surfaces as does the present invention, and generally the prior art devices involve complex assemblies which include devices or surfaces which tend to score the cylinder wall or piston surface or involve significant additional friction in operation.